Mid Sweden University

Modification of softwood kraft pulp by the addition of either polysulfide (PS) or sodium borohydride (NaBH₄) has been shown to increase pulp yield due to a higher retention of hemicellulose. The modified pulps showed higher tensile index, especially at lower refining. It also showed a greater porosity of the fibre wall, indicating an increase in the swelling potential of the fibres thus helping to increase fibre flexibility, increase joint strength between the fibres and to raise the tensile index. However, the swelling increase associated with the higher hemicellulose content could also make dewatering more challenging because of the higher water retention of pulp. But recent studies showed the positive influence of increase pulp yield dominates over the negative influence of the higher hemicellulose content on dewatering properties, especially at lower refining. Studies simulating full-scale tissue dewatering conditions showed that pulps with higher hemicellulose content had a higher tensile index at the same dryness which was achieved in a shorter dwell-time. A given tensile index was also achieved with less refining energy. Therefore modification of the kraft pulping process is now a way to give high quality fibres for tissue paper production with less refining energy and lower drying energy costs.